This invention relates to secondary-aliphatic (alkyl, cycloalkyl and aralkyl) azoalkanes containing one .alpha.-substituent per azo group which have been found to be more thermally stable than corresponding tertiary azo compounds such as those disclosed in our Canadian Patent No. 924,299 and therefore to operate as vinyl polymerization initiators or curing agents for resins at higher temperatures.
It is well known in the polymer field that polymer properties can be altered considerably by the conditions under which the polymerization is run. Reaction temperature is often a critical variable in the polymerization reaction. Therefore it is important to have an initiator with the proper decomposition rate at the temperature the polymerization is run. Often polymerizations are run with several initiators of widely varying half-life present. For instance, a low temperature initiator may be employed to generate a high concentration of free radicals to initiate the polymerization and provide an exotherm, a medium temperature initiator may then be employed to generate free radicals through the bulk of the polymerization and finally a high temperature initiator used to finish off the polymerization. Similar mixtures can be used to cure polyester resins. A low temperature initiator can be used to initiate the gelling and curing in a short time or at a low temperature. This will generate enough of an exotherm to kick-off the high temperature initiator which will provide the hard cure. Therefore it is advantageous to have a series of initiators available with a wide range of stabilities so that specific industrial needs may be met. The addition of the series of sec-alkyl-.alpha. -substituted azoalkanes to the already known t-alkyl-.alpha. -substituted azoalkanes will increase the operating ranges that polymerizations can be carried out using azo initiators.
We are not aware of any prior art reporting compounds as disclosed and claimed herein, although many secondary alkyl azoalkanes have been reported in the literature. For example, J. A. Berson et al, J.A.C.S. 84, 3337 (1962), studied the decomposition of 2-azobornane, azoisopropane and azoethane. S. G. Cohen et al, J.A.C.S. 72, 3947 (1950), studied the decomposition rates of 1-azo-bis-1-arylalkanes. Previous studies on such .alpha.-aryl-substituted azo compounds were also made by Thiele, Ber., 42, 2578 (1909); Lochte et al, J.A.C.S. 44, 2556 (1922); Schulze et al, J.A.C.S. 48, 1031 (1926); and Fodor et al, Ber., 76B, 334 (1943). R. C. Neuman, Jr. et al, J. Org. Chem., 35, 3401 (1970) studied the thermal decomposition of .alpha.-cumylazocyclohexane. L. Spialter et al, J. Org. Chem. 30, 3278 (1965) reported the preparation of unsymmetrical secondary alkylazoalkanes. C. G. Overberger et al, J.A.C.S., 81, 2154 (1959) studied the decomposition rates of symmetrical and unsymmetrical secondary azoalkanes, finding that only those compounds containing an .alpha.-phenyl group decomposed at an appreciable rate at 120.degree. C. or below. Cyclic azoalkanes have been reported by C. G. Overberger et al in J.A.C.S., 88, 658 (1964); J.A.C.S., 86, 5364 (1964), J.A.C.S., 80, 6556 (1958), J.A.C.S., 77, 4651 (1955) and J.A.C.S., 87, 4119 (1965); S. G. Cohen et al in J.A.C.S., 84, 586 (1962); and R. M. Moriarty in J. Org. Chem., 28, 2385 (1963). U.S. Pat. No. 3,350,385 reports secondary azoalkanes prepared by tautomerization of the corresponding hydrazone. Secondary-alkyl-.alpha.-hydroperoxy-azoalkanes are reported in our copending application U.S. Ser. No. 88,249, filed Nov. 9, 1970 (now abandoned, replaced by continuation-in-part application 453,445 filed Mar. 21, 1974). 2-(1-Phenyl-2-propyl)azo-2-acetoxypropane is reported by D. C. Iffland et al, J.A.C.S., 83, 747 (1961), such acyloxy compounds being too thermally stable to have practical utility as initiators or curing agents.